1. Field of the Invention
The field of the present invention relates in general to plasma reactors and processes typically used to process semiconductor substrates or the like. More particularly, the field of the invention relates to a shielded plasma reactor in which the plasma is generated primarily by inductively coupled power.
2. Background
The era of microelectronics, and more specifically of Very Large Scale Integration, has been built in large part around vacuum processing of semiconductors to obtain Integrated Circuits. One of the contributors in this field of vacuum processing, which enabled the development and success of today's semiconductor industry, is undoubtedly the outstanding parallel development of plasma tools and of related processing techniques. The plasma-based equipment market is a multi-billion dollar annual business, be it for thin film deposition, etching or even ion implantation.
The history of plasma etching is relatively short, starting only around twenty years ago. The first commercially available equipment featured a basic diode-like configuration. The plasma was generated in a vacuum chamber between two flat electrodes. One of the electrodes was typically used as a wafer susceptor, and was also usually connected to a high-frequency power supply. The other electrode was typically grounded, although in some configurations the second electrode was connected to a high frequency power supply instead of or in addition to the first electrode. This configuration was very successful at the time, but important technological limitations started to become evident in the early eighties. As integrated circuit geometry continued to shrink dramatically, the semiconductor industry sought to develop plasma equipment capable of producing dense plasmas with low energy ions. A dense plasma is important in order to achieve a high rate of processing, while low energy ions are important to avoid damaging small integrated circuit features which are susceptible to damage from bombardment of high energy ions. In order to produce a high density, low ion energy plasma, it is desirable to de-couple the control of plasma density from the control of ion energy in a plasma.
The first serious generation of decoupled, or advanced, plasma sources that appeared in the eighties relied on a microwave power source and featured a special magnet arrangement (technology known as Electronic Cyclotron Resonance, or ECR) capable of delivering a high plasma density without high energy ion bombardment of the semiconductor wafer. The usefulness of conventional ECR to address the evolving needs of the semiconductor industry is limited, however, mainly because of the complexity and very limited operating pressure range of conventional ECR.
A more promising approach has more recently emerged based upon the conventional Inductively Coupled Plasma, or ICP, which was originally invented at the end of the 19th century. Modem ICP sources, which have been improved and adapted for use in semiconductor processing, appear to provide a technology with the potential to meet the needs of the semiconductor industry well into the next decade. While ICP and ECR both provide high density, low ion energy plasmas, ICP allows a drastically wider pressure range to be used for processing. The pressure range for typical ICP sources ranges from about 0.5 mtorr to about 1 torr, whereas typical ECR sources are limited to an operating range of about 0.5 mtorr to about 5 mtorr. Consequently, ICP is suitable for a very wide range of applications that intrinsically require very different process pressures--from low pressure, fine pattern anisotropic etching to high pressure, isotropic etching.
Nevertheless, some conventional ICP sources suffer from a disadvantage in that they are prone to generate a significant amount of energetic ions. This is caused by the fact that the induction coil used to inductively couple energy into the plasma also causes some capacitive coupling of electromagnetic energy between the metal coil and the plasma (a phenomenon called parasitic capacitive coupling of the ICP inductor). In a typical ICP reactor, an induction coil surrounds a plasma production chamber below which a semiconductor substrate is placed for processing. Radio frequency power is applied to the induction coil and thereby inductively coupled into the plasma production chamber. The inductively coupled power accelerates ions circumferentially in the plasma substantially parallel to the semiconductor substrate. While inductively coupled power from the induction coil tends to accelerate ions in a plane parallel to the semiconductor substrate, the parasitic capacitive coupling tends to accelerate ions radially outward from the plasma which causes high energy ions to bombard the semiconductor substrate below the plasma. This problem is described in detail in U.S. Pat. No. 5,534,231 the disclosure of which is hereby incorporated herein by reference in its entirety.
This parasitic capacitive coupling can be substantially blocked by using a split electrostatic shield, also known as a split Faraday shield, positioned between the induction coil and the dielectric plasma chamber wall. The shield substantially blocks capacitive coupling while allowing inductive coupling of power into the plasma. Conventional split electrostatic shields typically comprise metal plates or a metal cylinder forming longitudinal slits transverse to the induction coil. The metal body of the shield blocks capacitive coupling, while the slits allow inductively coupled power to penetrate the shield. The slits prevent circumferential current loops from forming in the shield which would otherwise substantially prevent the penetration of the inductive electric field. Such electrostatically shielded ICP reactors are described in U.S. Pat. No. 4,918,031, U.S. Pat. No. 5,234,529 and U.S. Pat. No. 5,534,231, each of which is hereby incorporated herein by reference in its entirety. The excellent performance of such electrostatically shielded ICP sources demonstrates the promising potential of this technology for use in future semiconductor plasma processing equipment. Nevertheless, conventional split electrostatic shields are not ideal for all plasma processing. The slits allow some capacitive coupling through the slits which may in turn cause non-uniform power deposition in the plasma. In addition, conventional electrostatic shields typically comprise relatively complex, bulky and expensive solid-metal structures that must be fitted and supported around the outside of the dielectric ICP chamber wall.
What is needed is an electrostatic shield for ICP semiconductor processing reactors which provides more uniform and continuous shielding around the plasma. What is also need is an electrostatic shield that provides improved shielding of capacitive electric fields while allowing virtually unimpeded penetration of inductive electric fields. Preferably such an improved shield will be inexpensive and easy to manufacture and deploy.